Electron-beam and UV-cured adhesives are currently the most rapidly growing segments of the radiation-cured polymer market. Of particular commercial importance are UV-curable epoxide adhesive formulations, which typically consist of three principle components: i) cationic photoinitiators, ii) alcohols or polyols, and iii) epoxide monomers.
The photoinitiators are chemically-inert compounds that liberate acidic species upon exposure to actinic radiation. These acidic species then catalyze the crosslinking of the epoxide monomers. Typical photoinitiators include diaryliodonium, triarylsulfonium and ferrocenium salts.
Alternatively, it is possible to thermally initiate cure through the use of those onium or pyridinium salts that are known to afford cationic species capable of initiating cationic cure upon heating. For example, it is known that N-benzylpyridinium and related quaternary ammonium salts afford acidic species under thermolysis conditions (Lee, S. B.; Takata, T.; Endo, T., Macromolecules, 1991, 24, 2689-2693). It is also known that diaryliodonium salts thermally decompose in the presence of catalytic amounts of copper compounds (Crivello, J. V.; Lockhart, R. T. P.; Lee, J. L., J. Polym. Sci., Polym. Chem. Ed. 1983, 21, 97), and that these diaryliodonium salts can be converted to acidic species via decomposition of benzpinacol (Abdul-Rasoul, F. A. M.; Ledwith, A.; Yagci, Y. Polymer, 1978, 19, 1219-1223), or peroxides (Crivello, J. V.; Lam, J. H. W. Polym. Photochem. 1982, 2, 219). A recent report indicates that N-allyloxypyridinium salts can be thermally converted to acidic species in the presence of 2,2'-azobutyronitrile or benzoyl peroxide (Reetz, I.; Bacak, V.; Yagci, Y. Macromol. Chem. Phys. 1997, 98, 19-28). Any of these routes will liberate cationic species capable of effecting the ring-opening polymerization of the styrene oxides.
The alcohols or polyols act as a source of active protons, thereby facilitating the conversion of the photoinitiator to the cationic species, which activates the cationic polymerization. They also provide flexibility and impact resistance to the formulation through copolymerization with the epoxides.
The epoxide monomers used in these formulations are mainly cycloaliphatic epoxides, although glycidyl esters, glycidyl ethers and epoxidized alpha-olefins also have been used. The cycloaliphatic epoxides are the preferred compounds because they are more reactive than epoxides of straight chain aliphatics. It has been surmised that this greater reactivity is the result of two structural features: cleavage of either C--O bond leads to formation of a relatively stable secondary carbocation; and the cleavage of a C--O bond also releases the ring-strain associated with the bicyclic ring fusion.
The most common epoxide resin in UV-curable formulations is a bis-cyclohexene oxide (available from Union Carbide, product ERL-4221) connected by an ester group. This bis-cyclohexene oxide possesses sufficient reactivity to provide good crosslinking at ambient temperature. Moreover, the ester group is the only other functionality present and it is transparent to UV-radiation. However, there are drawbacks to this monomer. The bis-epoxide is an inherently non-flexible material and consequently produces a brittle crosslinked network. Such brittle materials are susceptible to mechanical stresses in manufacturing operations or end use applications. To counteract this, the epoxide can be co-reacted with one or more flexible diols in order to provide needed flexibility. However, the cycloaliphatic epoxides are not compatible with a particularly broad range of diols, which consequently limits the range of properties that may ultimately be achieved.
Although Crivello, et al. (Radiation Curing in Polymer Science and Technology, Vol.2, J. P. Fouassier and J. F. Rabek (Eds.), Elsevier Applied Science, New York, 1993, pp 435-472; Macromolecules, 1996, 29, 433-438 and 439-445; J. Polym. Sci., Polym. Chem. 1995, 33, 1881-1890) have reported several epoxide structures (e.g., epoxy norbornene and limonene oxides) that reputedly overcome some of the drawbacks of traditional cyclohexene oxides, there is a need for new monomers that are cationically curable and that avoid the problems of the cycloaliphatic epoxide monomers.